A magnetic recording medium using ferromagnetic metal particles with high saturation magnetization (.sigma.s) and high coercive force (Hc) has been recently researched and developed to improve recording density and reproducing output.
The following methods for preparing ferromagnetic metal particles have hitherto been known:
(1) an organic acid salt of ferromagnetic metal is hydrolyzed and then reduced with a reducing gas (see Japanese Patent Publication Nos. 11412/61, 22230/61, 14809/63, 3807/64, 8026/65, 8027/65, 15167/65, 12096/66, 24032/67, 3221/68, 22394/68, 29268/68, 4471/69, 27942/69, 38755/71, 4286/72, 38417/72, 41158/72 and 29280/73, Japanese Patent application (OPI) No. 35823/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and U.S. Pat. Nos. 3,186,829 and 3,190,748);
(2) an acicular oxyhydroxide of a ferromagnetic metal, an acicular oxyhydroxide of a ferromagnetic metal and another metal, or acicular iron oxide derived from these oxyhdroxides is reduced with a reducing gas (see Japanese Patent Publication Nos. 3862/60, 11520/62, 20335/64, 20939/64, 24833/71, 29706/72, 39477/72, 24952/73 and 7313/74, Japanese Patent application (OPI) Nos. 7153/71, 38523/72, 79153/73, 82395/73 and 97738/74, and U.S. Pat. Nos. 3,598,568, 3,634,063, 3,607,219, 3,607,220 and 3,702,270);
(3) a metal carbonyl compound is thermally decomposed (see Japanese Patent Publication Nos. 1004/64, 3415/65, 16968/70 and 26799/74 and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882);
(4) a ferromagnetic metal is vaporized in a low-pressure inert gas (see Japanese Patent Publication Nos. 25620/71, 4131/74, 27718/72, 15320/74 and 18160/74 and Japanese Patent application (OPI) Nos. 25662/73, 25663/73, 25664/73, 25665/73, 31166/73, 55400/73 and 81092/73);
(5) a metal salt capable of forming a ferromagnetic material in aqueous solution is reduced with a reducing material (e.g., borohydride compound, hypophosphite or hydrazine) to form ferromagnetic particles (see Japanese Patent Publication Nos. 20520/63, 26555/63, 20116/68, 9869/70, 14934/70, 7820/72, 16052/72 and 41718/72, Japanese Patent application (OPI) Nos. 1363/72, 42252/72, 42253/72, 44194/73, 79754/73 and 82396/73, U.S. Pat. Nos. 3,607,218, 3,756,866, 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,669,643, 3,672,867, and 3,726,664 and Japanese Patent application Nos. 91498/73, 92720/73, 106901/74 and 134467/73); and
(6) particles of a ferromagnetic metal are electro-deposited on a mercury cathode from which the particles are then separated (see Japanese Patent Publication Nos. 12910/60, 3860/61, 5513/61, 787/64, 15525/64 and 8123/65, and U.S. Pat. Nos. 3,262,812, 3,198,717 and 3,156,650).
The invention relates to a method for preparing ferromagnetic metal particles in accordance with the above method (2).
Coercive force (Hc) of ferromagnetic metal particles generally depends upon the anisotropy of the acicular shape of particles, and it is important to maintain the acicular shape. However, there is a problem with method (2) in that as reduction is carried out in a hydrogen gas at a high temperature, so sintering easily occurs in the reducing step. In order to eliminate this problem, there has been proposed in Japanese Patent application (OPI) No. 63605/82 a method which comprises attaching or adsorbing a compound which is capable of preventing sintering on the surface of acicular iron oxyhydroxide particles, then dehydrating acicular iron oxyhydroxide particles in a non-reducing gas under heating and reducing the resulting acicular iron oxide particles in a reducing gas under heating.
However, since the compound attached on the surface of iron oxyhydroxide particles comes into the inside of iron oxyhydroxide particles in some amount, an acicular shape can hardly be maintained. Accordingly, the shape of thus prepared metal particle is readily broken. Further the crystal size of the skeltone is large and thus its specific surface area is small. The noise level of signals of a magnetic recording medium using ferromagnetic metal particles having a large crystal size is high. Therefore the large crystal size is not preferred.